Flexible electronics is a field of research in rapid expansion, driven by high hopes for novel applications such as electronic journals, RFID (Radio Frequency Identification Tags) type data transmission tags, or reconfigurable displays. A review of these potential applications is described in the article “Organic and polymer transistors for electronics” (A. DODABALAPUR Materials today, volume 9 No. 4, 24 (2006).
A solution widely used in the flexible electronics field has for a long time consisted of using polymers such as Poly(p-phenylenevinylene) PPV or Poly(3-hexylthiophene) P3HT and small organic molecules of the pentacene or rubrene type.
The article “Megahertz operation of organic field-effect transistors based on poly(3-hexylthiophene)” (V. WEIGNER et al.—Applied Physics Letters volume 89 No. 24, 243515 (2006)) describes a field effect transistor with a poly(3-hexylthiophene) polymer film channel with a unity gain frequency of 2 MHz, the mobility of charges in this polymer being limited to 0.2 cm2/V.s.
In addition, the article “Organic complementary D flip-flops enabled by perylene diimides and pentacene” (B. YOO et al.—Institute of Electrical and Electronics Engineers Electronic Device Letters volume 27 No. 9, 737 (2006) describes the manufacture of a complementary D flip-flop electronic device including organic CMOS semiconductor transistors. The n and p type organic semiconductors used are, respectively, N,N′-bis(n-octyl)-dicyanoperylene-3, 4:9, 10-bis(dicarboximide) (PDI-8CN2) and pentacene. The devices presented operate at a clock speed of 5 kHz. Transistors in PDI-8CN2 and in pentacene have very low mobilities (6.3.10-2 and 0.29 cm2/V.s respectively) and, in addition, operate under very high electrical bias voltages (+100 and −100 volts respectively).
By way of example, the article “Radio frequency rectifiers based on organic thin-film transistors” (R. ROTZOLL et al.—Applied Physics Letters volume 88 No. 12, 123502 (2006) may also be cited, which presents a power rectifier utilizing P-MIS (Metal Insulator Semiconductor) type transistors with a semiconductor layer of pentacene on a polyethylenenaphthalate substrate operating up to 20 MHz, the mobility of the charges in the pentacene layer being less than 0.3 cm2/V.s.
In addition, the article “A 13.56-MHz RFID system based on organic transponders” (E. CANTATORE et al.—IEEE Journal of Solid-State Circuits volume 42 No. 1, 84 (2007) presents a radio frequency identification device constructed on a flexible sheet of polyimide operating at 13.56 MHz, the mobility of charges being estimated at 10-2 cm2/V.s.
Thus it is observed that in spite of the large efforts undertaken, the relatively low mobility of charges in these materials (10-3-10 cm2/V.s) very strongly limits applications with high frequencies of operation.
A solution to allow applications with a higher frequency of operation consists of utilizing semiconductor nanowires and ribbons, which are materials that also enable flexible electronic devices to be made.
Thus, the article “Gigahertz operation in flexible transistors on plastic substrates” (Y. SUN et al.—Applied Physics Letters volume 88 No. 18, 183509 (2006) describes a device on a poly(ethylenetherephthalate) plastic substrate based on GaAs semiconductor wires with an operation frequency of 1.55 GHz. The authors demonstrate that these devices operate in tension and in compression up to levels of approximately 0.71% (which corresponds to a radius of curvature of 14 mm). The current comes back to its initial value when the constraints are released. Under constraints greater than 1%, the devices undergo irreversible damage which, according to the authors, is very likely caused by a rupture in the wires or gate electrodes. The angles of curvature reached in this work therefore remain modest, which constitutes a strong limitation for the conceivable applications.
In addition, the article “High-speed mechanically flexible single-crystal silicon thin-film transistors on plastic substrates” (J. H. AHN et al.—IEEE Device Letters volume 27 No. 6, 460 (2006) describes transistors on a plastic (polyimide) substrate for which the active part consists of single crystal silicon ribbons. The continuous (DC) and high frequencies responses of the devices reveal a mobility of 500 cm2/V.s and an operation frequency that reaches 515 MHz. In addition, its operation in flexion is stable up to 3 mm radii of curvature. However, as for devices based on organic polymers and materials, the relatively low mobility of the charges in silicon also limits the potential applications at high frequency.
More recently, research in the field of flexible electronics has also been devoted to the use of carbon nanotubes.
Several studies have been devoted to the static performances of flexible transistors utilizing carbon nanotubes. In these studies, the authors associate a study of the electrical characteristics of the device when it is under tension and/or compression constraints. As an example, the article “Highly bendable, transparent thin-film transistors that use carbon-nanotube-based conductors and semiconductors with elastomeric dielectrics” (Q. CAO et al.—Advanced Materials volume 18, 304 (2006) may be cited. This article describes transistors obtained by transfer of different layers of single-wall carbon nanotubes on a sheet of poly(ethyleneterephthalate). These devices support flexions up to a level of approximately 2% with recovery of the initial transconductance and current level characteristics when the constraints are removed. However, only static performance is described in the case of this device and high frequency applications are not proposed.
More recently, the article “High-speed thin-film transistor on flexible substrate fabricated at room temperature” (J. VAILLANCOURT et al.—Electronic Letters volume 42, 1365 (2006) described the design of a thin film transistor of carbon nanotubes on a flexible substrate operating around 150 MHz at low electrical bias voltage (VDS=2 V). In this work, the nanotubes are deposited in the form of films by the spin coating technique. Here, the nanotubes are deposited in a disorganized manner, which strongly limits the high frequency performances.
In addition, the article “High-performance electronics using dense, perfectly aligned arrays of single walled carbon nanotubes” (KANG et al.—Nature Nanotechnologies—25 Mar. 2007) described a growth method for self-aligned (versus “deposited in a disorganized manner” as described in the previous paragraph) nanotubes on a rigid crystal substrate and the utilization of this method to manufacture flexible devices by transfer of nanotubes from the rigid growth substrate to a flexible substrate.
Therefore, the implementation of the method according to the article “High-performance electronics using dense, perfectly aligned arrays of single walled carbon nanotubes” also poses some difficulties.
The major disadvantage of this method resides in obtaining a low mobility of the components (on the order of 480 cm2/V.s). This low mobility does not enable high frequency work on the flexible substrate.